RAMAN-SPECTROSCOPIC MEASUREMENT OF OXIDATION IN SUPERCRITICAL WATER .1. CONVERSION OF METHANOL TO FORMALDEHYDE

The oxidation rate of methanol and the subsequent production and destr uction of the primary intermediate, formaldehyde, were investigated us ing Raman spectroscopy as an in situ analytical method. Experiments we re conducted in supercritical water over temperatures ranging from 440 to 500 degrees C at 24.1 MPa and at a nominal feed concentration of 0 .05 mol/L (1.5 wt %). Effluent samples were also examined using gas ch romatography. In these experiments, feed concentrations ranging from 0 .011 to 1.2 wt % and temperatures from 430 to 500 degrees C were exami ned and showed that the effective first-order reaction rate for the ox idation of methanol is dependent on the initial feed concentration. Ra man measurements reveal a temperature-dependent induction period of le ss than 1 s over the range of conditions investigated. In addition, qu antitative measurements of the production of formaldehyde indicate it is a key metastable intermediate. An elementary reaction mechanism, wh ich reproduces accurately the quantitative features of methanol oxidat ion and formaldehyde production, is used to identify key rate controll ing reactions during the induction period and the transition to the pr imary oxidation path.